A gamma-ray interacting with a detector loses energy to the detector with each interaction, such that a charge builds up in the detector which is proportional to the energy lost by the gamma-ray. Such detectors are commonly used to measure the total energy deposited by incident gamma-rays. To accomplish this purpose, a simple system measuring the amount of charge produced in the detector can be utilized in conjunction with the detector.
However, some physics experiments and detector technologies require more detailed information about gamma-ray interactions with the detector than these simple systems can provide. For example, where the energy deposited by individual gamma-rays is to be measured, experimenters need further data to account for effects such as Doppler shifts and Compton scattering.
Doppler shifts result from the well-known Doppler effect where the change in measured frequency of a wave at a given measuring point is dependent upon the motion of the source of the wave relative to the measuring point. The degree to which one can correct gamma-ray measurements in a detector for Doppler shifts depends upon the accuracy of the measurement of the position at which the gamma-ray first interacted with the detector.
Historically, the efforts made to improve position certainty centered upon reducing the size of individual detector elements composing a detector array. The level of position certainty was limited to the physical size of each individual detector element in the array.
A need for greater position certainty has led to the concept of segmented semiconductor detectors. In a segmented detector, the detector is divided into a number of segments one or both of the electrodes is physically divided into a number of segments (thereby providing in actuallity a plurality of separate electrodes). A separate signal is measured from each segment. The segment closest to the location where a gamma-ray interacts will display a signal with a non-zero integral. Adjacent segments will display induced signals with an amplitude that increases with the nearness of the interaction to the electrode; these signals have a zero integral. The signals from all of the segments must be analyzed together in order to determine the coordinates of the location or locations where the gamma-ray interacted. The induced signals provide the coordinates perpendicular to the detector's electric field. The coordinate parallel to the electric field, herein called the radial coordinate, can only be obtained by performing a detailed pulse shape analysis of the signal that has a non-zero integral.
Prior efforts in this field have obtained only rough determinations of the radial coordinate. The best prior efforts [Th. Kroll, I. Peter, Th. W. Elze et al., Nuclear Instruments and Methods in Physics Research A371, 489 (1996)] have obtained radial positions to an uncertainty of 4 mm to 8 mm. However, the accuracy is limited by using a simple measure of the average rise time of the signal, regardless of the fact that the signal may correspond to multiple gamma-ray interactions, each with its own radial location and corresponding rise time. In contrast the method of the present invention utilizes the entire signal in order to obtain the radial location of each interaction with an uncertainty of 1.0 mm and less (depending upon deposited energy), even for multiple gamma-ray interactions.
When all three coordinates of the gamma-ray interactions are measured with sufficient accuracy such as in the present invention, new detector technologies become possible. For example, one can utilize a segmented semiconductor detector and the full signal processing techniques to obtain an image of an object that emits gamma-rays. This is an extension of the concept of Compton imaging, which is well known in the scientific literature. Using a large segmented germanium detector, one can obtain such images with high efficiency, with excellent energy resolution, without a detector array, without collimation, and without tomographic techniques. The existing art has trade-offs in all of these areas. There is no known existing art that encompasses all of these properties.
Another new detector technology provided by the present invention is the ability to perform Compton suppression in gamma-ray spectroscopy without the use of anti-coincidence detectors. For gamma-rays in the energy range of roughly 0.15 MeV to 8 MeV the Compton interaction is the dominant mode by which a gamma-ray interacts with germanium. In a Compton interaction the incident gamma-ray interacts with an atomic electron. The electron receives some of the gamma-ray's energy, and the remainder is conserved in a scattered gamma-ray with less energy than the original gamma-ray. If the scattered gamma-ray escapes from the detector, the event is called a "Compton escape event".
An escape event can skew the measurement of the spectrum of gamma-rays incident upon a detector because the energy deposited in the detector represents only a part of the energy of the original gamma-ray. If not identified and accounted for, escape events not only provide inaccurate information about the energy carried by the original scattered gamma-ray, but may confuse measurements for other gamma-ray interactions with the detector. For example, measurement of a low energy gamma-ray line may be distorted by the presence of a large Compton-escape continuum from a higher energy gamma-ray unless the Compton escape events produced by the higher energy gamma-ray are suppressed from the measurement.
The traditional method of performing Compton suppression has required experimenters to surround the detector with anti-coincidence detectors. These detectors add cost, volume, and mass to detector measurement systems. While some work has been done to perform Compton suppression without the use of anti-coinicidence detectors, all such work has analyzed the signals produced by the detector for the average rise time of the signal.